Closed loop multiple-input-multiple-output (MIMO) systems typically transmit channel state information from a receiver to a transmitter. Transmitting the channel state information consumes bandwidth that might otherwise be available for data traffic.
Illustratively, conventional frequency division duplex (FDD) systems that employ beamforming (or, closed loop multiple input, multiple output (MIMO), the beamforming matrix (referred to herein as a codeword) generated in response to perceived channel conditions is computed and quantized at the receiver first, and then is provided to the source transmitter (e.g., via feedback). A conventional approach to reduce the overhead associated with this feedback is to provide matrix codebook(s) at each of the transmitter and the receiver, each of the codebook(s) comprising a plurality, or set, of potential beamforming matrixes that may be used depending on the channel conditions perceived at the receiver. When the receiver has identified the appropriate matrix codebook(s), the receiver will typically feed back only an index (instead of the actual matrix entries) that points to the appropriate codeword in the codebook(s) stored at the transmitter.
Thus, for a different combination of transmit antenna(e) (Nt) and data streams (Ns), a different matrix codebook is required. Conventionally, the size of the codebook is based on the number of transmit antennae and the number of data streams: Nt×Ns. For some systems, e.g., one implementing the developing 802.16e1, Nt and Ns are currently less than five (5) but are likely to increase to eight (8). Therefore, a substantial number of Nt by Ns combinations are anticipated, requiring a significant amount of memory within mobile communication devices in order to store such a large number of codebooks. 1See, e.g., the ANSI/IEEE Std 802.16-2001 Standard for Local and Metropolitan area networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems, its progeny and supplements thereto (e.g., 802.16a, .16d, and 16e).